Numerous prior art patents describe various pressure swing adsorption processes for selective adsorption of nitrogen from ambient air whereby an oxygen-enriched product stream is recovered. While such oxygen enriched air has beneficial use in many diverse applications, essentially nitrogen free oxygen product is needed for some chemical processes, medical, welding and other uses. However, most known PSA processes can efficiently generate only up to about 80-93% O.sub.2 product from ambient air because the efficiency of production decreases inordinately when higher purity oxygen product is demanded.
Where oxygen purity of about 95% or higher is desired resort must be had to purchase of liquid oxygen (LOX) or use of cryogenic means for recovery of oxygen from air. These alternatives require in the first instance transportation of the LOX to the site of use, and the latter instance cryogenic generation of the high purity oxygen at the site is often unattractive from an economical standpoint unless the plant size is sufficiently large.
PSA cycles have also been deviced for production of high purity oxygen by two-stage operating techniques whereby an oxygen-rich stream is first produced from ambient air followed by further removal therefrom of remaining nitrogen and argon impurities. Such two-stage operations are cumbersome, however, requiring many adsorbers and complex piping, in addition to which the overall efficiency of separation is quite low.
Among the various prior art patents directed to or disclosing PSA processes for production of oxygen-enriched product gas, only U.S. Pat. No. 3,717,974 contains an example directed to production of a product of 95% O.sub.2 purity. Zeolitic adsorbents used for production of oxygen and nitrogen from air selectively adsorb nitrogen from the air feed but do not exhibit any selectivity between oxygen and argon. Consequently, the maximum possible O.sub.2 purity that can be achieved by a PSA process is 95.7% O.sub.2 (complete nitrogen removal). Thus, while certain of the prior art PSA schemes may be pushed to achieve or approach production from air of a gas product having about 95% O.sub.2, such result can be achieved only at the cost of poorer performance. The performance of the PSA process deteriorates as the purity of the oxygen product is increased; both the specific production capacity (in terms of lbs. of product oxygen/lb. of zeolite/cycle), as well as the oxygen recovery, decrease with increase in O.sub.2 purity. As a result most PSA processes are designed to product about 85-92% O.sub.2 as an optimum product.
Typical PSA multi-column systems for fractionation of ambient air and recovery of an effluent enriched in oxygen and/or in nitrogen are disclosed in U.S. Pat. Nos. 3,923,477; 4,013,429; 4,065,272 and 4,329,158. While certain of these and other patents disclose adsorption of nitrogen from atmospheric air at superatmospheric pressure and vacuum desorption of the nitrogen-laden adsorbent bed, none of these (except the previously noted U.S. Pat. No. 3,717,974) discloses the production of a product having an oxygen content of 95% or above.
Among the several operating modes disclosed in the cited U.S. Pat. No. 3,717,974 the production of a product of .about.95% O.sub.2 purity is described only in Example 1 of the patent. To obtain the designated high purity O.sub.2 product, a nine step process is employed: adsorption, four depressurization steps, followed by a purge at near ambient pressure, then three repressurization steps. The .about.95% O.sub.2 product is obtained according to the example with a recovery of about 40% and at a production capacity of about 0.035 millipound moles/pound of the adsorbent. At the 70 psia adsorption pressure employed in the patent example the adsorption/desorption pressure ratio is about 4.8.
In accordance with the present invention, as hereinafter described, high purity oxygen product is obtained from atmospheric air at considerably higher yield than heretobefore by a simplified and cost efficient four step PSA operation, including adsorption at a moderate superatmospheric pressure and desorption at a moderate subatmospheric pressure within a prescribed adsorption/desorption pressure ratio.